Why flu season happens and how to help protect yourself

When is flu season?

In the Northern Hemisphere, including the United States, flu season typically begins in October, peaks between December and February, and lasts through April. However, the exact timing can vary by region and year.

In the Southern Hemisphere, flu season usually occurs from April to September, aligning with their winter months. In tropical regions, flu activity can occur year-round but may still have seasonal peaks.

Why do we have a flu season?

Several factors contribute to the seasonal nature of the flu:

• Colder weather: Flu viruses survive and spread more easily in cold, dry air.
• Indoor crowding: People spend more time indoors during colder months, increasing close contact and the chance of transmission.
• School sessions: Schools are crowded with both children and adults, who are at risk for the flu, and flu season typically coincides with the school year.
• Holidays: During holidays such as Thanksgiving, Christmas and New Years people often travel, have family get-togethers, and attend religious events, which can increase the risk of getting the flu from others.
• Weakened immune systems: Less sunlight in the winter can contribute to lower levels of vitamin D, which may affect immune function.

Are some flu strains worse than others?

Yes. Influenza viruses are constantly changing. Each year, different strains circulate—some more severe than others. The flu vaccine is updated annually to target the strains expected to be most common.

Some years, the dominant strain may cause more severe illness or spread more easily, which can lead to a more intense flu season.

Who gets the flu?

Anyone can catch the flu, but some groups are more vulnerable to complications:

• Adults 65 and older
• Young children, especially under the age of 5
• Pregnant women
• People with certain chronic conditions, such as asthma, diabetes, or heart disease
• People with weakened immune systems

Special Considerations

Some people may need special considerations:

• Older adults and adults with a solid organ transplant (e.g., kidney) (especially those 65 years and older) may benefit from high-dose or adjuvanted versions, which include an added ingredient to help strengthen the body’s immune response. The US Centers for Disease Control and Prevention specifically recommends one of these “enhanced” vaccines for older people and adults who have had a transplant.
• Pregnant individuals are encouraged to get vaccinated to help protect themselves and their unborn child. Pregnant women should discuss with their healthcare provider about receiving the flu vaccine earlier in the Fall.
• People with a weakened immune system should receive inactivated flu vaccines. Live attenuated vaccines (like the nasal spray) are generally not recommended.
• People with egg allergies can opt for egg-free formulations but the Centers for Disease Control and Prevention states that such people may receive any age-appropriate flu vaccine.

It’s also important to note that many people use the term “flu” to describe any respiratory illness during the winter months. However, the flu vaccine only protects against influenza viruses. While there are vaccines for other respiratory viruses—like COVID-19 and RSV—not all respiratory illnesses are preventable by vaccination, including rhinovirus (common cold) and human parainfluenza viruses (HPIV).